1. Field of the Invention
The present invention relates to an image processing method and apparatus. For example, the present invention relates to a method of transferring image data obtained by a double-sided image scanner scanning on both sides of a double-sided document in parallel.
2. Description of the Related Art
When scanning image information of a double-sided document, a conventional image scanner with an automatic document feeder scans on the first side of the document at the scanning position, turns over the document, conveys the document to the scanning position again, and scans on the second side of the document.
However, some recent image processing apparatuses scan the image information of both sides of a document in parallel to improve the scanning productivity, suppress the jam ratio and document degradation, and reduce noise (JPA62-171261, JPA11-069044, and JPA2002-77596). In such an image scanner, a first image sensor for scanning the image information of one side of a document and a second image sensor for scanning the image information of the other side of the document are provided in the document feeding path. The second image sensor is arranged downstream (or upstream) in the document conveying direction with respect to the first image sensor.
The image scanner capable of scanning in parallel on both sides transfers image data in the following way. Scanned data of the first side is transferred in real time. Scanned data of the second side is temporarily stored in a memory, and after the transfer of the image data of the first side is ended, transferred at a higher speed by increasing a speed of the transfer clock (especially JPA62-171261).
However, the speed of the transfer clock must be more increased because the second side data transfer must be done in a short time between the end of the first side data transfer and the start of next document scanning. An image processing unit in a controller unit on the receiving side compresses the received image data by enlarging the received image under resolution conversion. In data transfer at a higher clock speed, therefore, the processing of the image processing unit is delayed depending on the set enlargement ratio, and thereby an overrun error occurs.
This problem can be solved by temporarily storing data in a memory of the controller unit and then sequentially reading out the data from the memory for image processing. However, this adds the cost of the memory and also deteriorates the performance because of the loop-back generated by intervention of the memory.
On the other hand, the above problem may also be solved by switching the transfer clock between a mode with enlargement by the image processing unit and another mode without enlargement (non-scaled processing). This adds the cost corresponding to clocks for the enlargement process in the image processing unit. If the clock speed of second side data transfer is consistent with the throughput of image processing, the performance in non-scaled processing to be normally used degrades largely.